This invention relates generally to digital lightwave communications systems and, in particular, to methods and systems for modulating the output of a digital lightwave communications system. Even more particularly, the present invention relates to a method and system for first-order radio frequency (xe2x80x9cRFxe2x80x9d) amplitude and bias control of a modulator.
One of the most important competitive characteristics of a lightwave transmission system is how large a distance can be spanned between a receiver and transmitter while maintaining the integrity of the transmitted data. Such systems can be limited by the output power of the transmitter or by the receiver performance characteristics, specifically receiver sensitivity. The method of modulating the digital output from a transmitter can also greatly influence the distance separating the transmitter from the receiver. Modulating a digital lightwave output generates the digital xe2x80x9c1xe2x80x9d""s and digital xe2x80x9c0xe2x80x9d""s that are transmitted, and hence determines the content and integrity of the digital signal. From an economic viewpoint, the distance that can be spanned between a transmitter and receiver, while maintaining data integrity, determines the expenditures that must be made to physically lay fiber in the ground or to install repeaters and other supporting equipment.
One way to control the output of a digital lightwave communications system is to directly modulate the laser light source. For example, the laser could be turned on and off at intervals, thus generating digital 1""s (on) and digital 0""s (off). This can be accomplished by turning the current to the laser on and off. While this method may work in lower speed digital communications systems, in high-speed digital lightwave communications it is not practical to directly modulate the output of the laser because, as the current to the laser is changed, the wavelengths of the laser outputs are also slightly changed.
Direct laser modulation could thus cause significant dispersion in each of the different wavelengths traveling along a fiber optic cable, resulting in noise and data corruption at the receiver end of a high-speed digital lightwave system. This is because, particularly in a directly modulated laser system, multiple wavelengths are introduced by the modulation process. Each of these wavelengths has a slightly different propagation time, resulting in overlap at the receiver and therefore in possible data corruption and/or loss. In WDM (wavelength division multiplexing) systems, a significant amount of noise also results from carrying multiple wavelengths on a single fiber. This can result in loss of receiver sensitivity, because it is more difficult for the receiver to distinguish between the digital 1""s and 0""s, and hence to interpret the data carried by the signal.
High speed digital lightwave communications systems instead use modulators to modulate the laser output. Modulators do not affect the wavelengths carrying the data signal as much as direct modulation. However, these modulators require an RF amplitude input and bias point that must be set and maintained at or near an optimum value for each modulator. Otherwise, the resulting wavelength shift in the transmitted data, along with the inherent noise and dispersion occurring in WDM transmission systems, can result in the signals received at the receiver being noisy and difficult to differentiate.
Every modulator, such as a Mach-Zehnder modulator, can have a slightly different optimum RF amplitude input voltage (a peak-to-peak voltage Vxcfx80) and a slightly different optimum bias point (voltage). Together this optimum bias point and optimum RF amplitude Vxcfx80 provide the best extinction ratio (the ratio between full light output to no light output) for the modulator. As the RF amplitude level drifts away from optimum (Vxcfx80), the received signal becomes noisier, resulting in increased difficulty for the receiver in differentiating the 1""s and 0""s that comprise the signal. Similar signal degradation also occurs with changes in the bias point away from its optimum value.
Current high-speed digital lightwave transmission systems manually set the RF amplitude as close as possible to the optimum value Vxcfx80 and have no mechanism for automatically maintaining the RF amplitude at or near its optimum point. This xe2x80x9cset-it-and-forget-itxe2x80x9d methodology cannot compensate for changes in the RF amplitude that might occur over time. For example, as temperature changes, the Vxcfx80 of the modulator will likely also change, as will the gain of the RF amplifier. An RF amplifier gain that is set and forgotten can thus change over time due to shock, vibration and/or changes in temperature, resulting in a received signal that is fuzzy and difficult to differentiate by the receiver. Also, the possibility exists that the RF amplitude may initially be set too high or too low due to operator error. As a result, the RF amplitude of the circuit cannot track changes within the modulator, or in the RF train, over temperature and time.
Another problem with current high-speed digital lightwave transmission systems is that the bias value is controlled by dithering the bias signal itself. Dithering of the bias signal to control bias value leads to what are called second-order effects in the dither. This means that, for example, if the bias is dithered at 500 Hz, then the output that must be looked for to control the bias value occurs at 1,000 Hz (i.e., the second harmonic of the dither signal). These second-order effects are much lower in amplitude than first-order (direct relationship) effects. Second-order effects can be analogized to the second derivative of a signal, which most closely approaches a flat line. The result of using second-order effects to control the bias is that prior art circuits must use a much greater amount of dither. It also means that the bias control circuitry must be much more sensitive and complex to identify the output control signal. A second-order circuit also generates more noise and tends to wander (is more imprecise than a first-order circuit).
Prior art second-order circuits are also not very robust, in that they are not high-gain circuits that can differentiate well between signal and noise, resulting in a circuit that is very susceptible to noise. These second-order circuits thus have a poor signal-to-noise ratio and have difficulty in differentiating and extracting the loop feedback signal from the noise.
Therefore, a need exists for a method and system for controlling the RF amplitude and bias value of a modulator using first-order linear effects.
A further need exists for a method and system for first-order RF amplitude and bias control of a modulator that uses a feedback loop to control RF amplitude, so that RF amplitude can track changes in the modulator and in the RF train, over temperature and time.
Still further, a need exists for first-order RF amplitude and bias control of a modulator having a high signal-to-noise ratio and comprising a robust circuit high bandwidth feedback loops.
The present invention provides a method and system for first-order RF amplitude and bias control of a modulator that substantially eliminates or reduces disadvantages and problems associated with previously developed methods and systems for RF amplitude and bias control of a modulator within a digital lightwave communications system.
In particular, the present invention provides a method and system for robustly (using first order effects) controlling the bias point and radio frequency (RF) amplitude level of a modulator for an optical transmitter. The method comprises the steps of extracting an output dither signal component of a digital optical output signal from the optical transmitter to drive a feedback loop; measuring the output dither signal component in the feedback loop for comparison to an input dither signal to the modulator; comparing the output dither signal to the input dither signal to determine their difference; and, based on the difference between them, maintaining the bias point and the RF amplitude level at an optimum value by varying an input voltage to the modulator via the feedback loop.
One embodiment of the system of this invention comprises a laser for providing an input light, a modulator to modulate the input light and generate a digital optical output signal, a radio frequency (RF) feedback loop to control an RF input voltage to the modulator, a bias feedback loop to control a bias input voltage to the modulator, an RF amplitude dither circuit to provide an RF input dither signal to the RF voltage input, and a bias dither circuit to provide a bias input dither signal to the bias voltage input. The modulator can be a Mach-Zehnder modulator.
The method and system for first order RF amplitude and bias control of a modulator of the present invention provides an important technical advantage in that it uses first-order linear effects to control the RF amplitude to and the bias value of a modulator.
A still further technical advantage of the present invention is that it provides a method and system for first-order RF amplitude and bias control of a modulator that uses a feedback loop to control the RF amplitude, so that the RF amplitude can track changes in the modulator and in the RF train over temperature and time.
An even further technical advantage of the present invention is that it provides a robust circuit for first-order RF amplitude and bias control of a Mach-Zehnder modulator with a high signal-to-noise ratio and high bandwidth feedback loops.